DOC Document backporting non-blocking sessions

[GuySartorelli]

I thought about saying something about how it can help normalise hosting between CMS 5 and CMS 6, but:

1. I couldn't think of a good way to word it that doesn't just add more confusion
2. There's no concrete plans to remove dynamodb as a required module for cloud hosting yet, so I don't want to open that can of worms before we have answers to likely questions

Issue

• Implement new file session handler module for Silverstripe CMS 5.x .github#401

[emteknetnz]

This PR for the changlog is fine

Should add a PR targetting 5.4 doc to mention this module is available - update the docs themselves similar to https://github.com/silverstripe/developer-docs/pull/733/files

[GuySartorelli]

I'll add those docs to the new module and make a PR to add them to the optional features section since it's not part of core.